<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>4.通过定位设置尺寸</title>
  <style>
    article {
      width: 400px;
      height: 400px;
      border: solid 8px blueviolet;
      margin: 100px;
      position: relative;
      overflow: hidden;
    }

    div{
      background: red;

      position: absolute;
      left: 300px;
      top: 100px;
      right: -100px;
      bottom: 100px;
      border-radius: 50%;
    }
  </style>
</head>

<body>
  <article>
    <div></div>
  </article>

</body>

</html>